I only use one plugin for iPhoto and it is Connected Flow's amazingly useful FlickrExport for iPhoto. I know some folks think it is a little pricey, but this plugin makes posting my pictures to Flickr a breeze, and I am willing to pay a fair price for anything that makes my digital life easier. From the Connected Flow blog comes news that FlickrExport (both for iPhoto and Aperture) has been updated to resolve an issue with uploading large amounts of pictures to Flickr. Full release notes are available for the iPhoto and Aperture versions here and here, respectively.
If you're a user of either plugin be sure to download the update. A license for FlickrExport for iPhoto costs £12 whilst the Aperture plugin will set you back £14.